This invention relates to fabrication of solar cells in general and, more particularly, to the production of dendritic silicon crystal webs from which individual solar cells are obtained.
Silicon crystals are typically grown in a growth furnace using a fused silica crucible to contain a silicon melt. Silicon dissolves in the crucible at a melt temperature of about 1412.degree. C. As a result, silicon monoxide (SiO) gas is evolved from the melt and condenses at the colder regions of the growth furnace and on the growing silicon crystals. In the case of Czochralski (CZ), cast, and some other crystal growth methods used to produce silicon wafers, the resulting wafers are chemically cleaned to remove the surface materials to a depth as great as 50-70 microns. In these cases, the SiO deposited on the grown silicon does not pose a problem. In other cases, such as the Dendritic Web growth process, the thickness of the silicon ribbon grown is very small, on the order of about 100 microns. Therefore, extensive removal of silicon cannot be tolerated for thin crystals.
The Dendritic Web silicon growth technique is well known and is described in the literature. In general, a thin ribbon, typically about 100 microns thick, is grown from silicon melt contained in a fused silica crucible. Silicon monoxide evolved from the crucible deposits on the growing ribbon and inside the furnace chamber. The oxide thickness ranges from about 100 to about 1000 .ANG.. Fourier Transform Infrared Analysis has shown that the oxide is composed of complex SiO.sub.x, where x ranges from 1 to 2. This oxide film must be removed for further processing of the crystal into solar cells. While some portions of the oxide can be removed by chemical cleaning with an acid solution, such as hydrofluoric acid (HF), such simple immersion does not completely remove the oxide.
In the past, it has been considered necessary to use mechanical cleaning methods to completely remove the oxide after hydrofluoric acid cleaning. This method is still not satisfactory since submicron SiO particles embedded in the silicon crystals cannot be completely removed. Although a strong acid mixture consisting of hydrofluoric and nitric acid can remove the oxide, this process consumes a significant amount of silicon and leaves a pitted surface, which is undesirable for thin crystals. In addition, this process is expensive for solar cell applications due to the cost involved in disposing of the hazardous chemicals.
In the field of silicon wafer technology, Czochralski, Float Zone, and Cast silicon wafers are typically sliced to the desired thickness using a diamond saw. This slicing operation damages the silicon surface to a depth of about 50 to 70 microns. This saw damage is typically removed using an acid mixture containing hydrofluoric acid (HF) and nitric acid (HNO.sub.3). To obtain a smooth surface and reduce the etching rate, acetic acid (CH.sub.3 COOH) is usually added to this mixture. This acid mixture, known as an isotropic etchant, removes silicon uniformly independent of the orientation of the crystal.
Caustic solutions, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), have also been used to remove silicon surfaces by chemical etching. However, the etch rate is dependent upon the temperature and concentration of the solutions as well as on the crystallographic orientation of the silicon. For example, the etch rate of crystal with {111} orientation is about 5 to 10 times slower than the etch rate of crystal with {100} orientation. In addition, the etch rate for {111} orientation significantly decreases as a function of time or the amount of material removed. It has been reported that within 1.5 mils total removal, the average rate drops to &lt;1/50 the starting rate and the later rate to &lt;1/200.